What causes it?
Judging from the amount of vegetation hanging off the bike, I'd say an unintended interface with the scenery. You can break dérailleurs off by smacking them on a tree, or you can let the spokes rip them off due to poor set up or flexy wheels, but that usually leaves its mark on the wheel too.
@gbj_tester started
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Judging from the amount of vegetation hanging off the bike, I'd say an unintended interface with the scenery. You can break dérailleurs off by smacking them on a tree, or you can let the spokes rip them off due to poor set up or flexy wheels, but that usually leaves its mark on the wheel too.